The antiphospholipid antibody syndrome (referred to as APS hereinafter) is an autoimmune disease with clinical manifestations including arterial or venous thrombosis, abortion and stillbirth, in which autoantibodies called antiphospholipid antibodies are identified in blood. According to the definition of APS, identification of antiphospholipid antibodies (referred to as aPL hereinafter) is essential for diagnosis. However, aPLs constitute an extremely diverse autoantibody group, and detection of them is not necessarily easy. Draft of amended APS classification criteria was prepared in 1999, and the draft included the following descriptions regarding test items:
1) moderate or higher level of IgG or IgM-type β2 glycoprotein I-dependent anticardiolipin antibodies; and
2) positive result for LA
However, it is said that the LA test is one of the most difficult tests for both of clinicians and clinical technologists, and it is known that results thereof vary to a large extent depending on test methods, reagent types and specimen conditions.
LA is defined as “immunoglobulins that inhibit in-vitro phospholipid-dependent coagulation reactions (reactions for activated partial thromboplastin time (Triplett LA. Lupus 7 (Supple. 2), S18-22, 1998; also referred to as aPTT hereinafter), kaolin clotting time (Triplett LA. Lupus 7 (Supple. 2), S18-22, 1998; also referred to as KCT hereinafter) and dilute Russell's viper venom time (Triplett LA. Lupus 7 (Supple. 2), S1B-22, 1998; also referred to as dRVVT hereinafter))”, and the Committee for Antiphospholipid Antibody standardization of Xnternational Society on Thrombosis and Hemostasis provided guidelines of testing in 1995.
Various means have been devised in each institution to increase sensitivity and specificity. However, it is still difficult to determine the presence of LA by a single method, and several methods are usually employed in combination for the determination. That is, these are used the steps of:
1) screening for prolongation of the phospholipid-dependent coagulation time with aPTT, KCT and dRVVT;
2) demonstrating that this prolongation of the coagulation time is resulted from the presence of an inhibitor in plasma of patient by the mixing test (addition of normal plasma); and
3) confirming that this inhibitor is an antiphospholipid antibody by an absorption and neutralization test using impaired platelets or phospholipid.
Since LA inhibits phospholipid-dependent reactions, reduction of the phospholipid concentration is required for highly sensitive detection of LA. However, even under such a condition, selection and preparation of reagents and determination of standard values need to be done in each institution, and thus there is a problem that results still vary to a large extent among institutions. Accordingly, quantitative LA assay is not generally conducted.
Regarding an index associated with LA, Atsuni T et al., Arthritis Rheum 43: 1982-93, 2000 reported that a phosphatidylserine-dependent antiprothrombin antibody (referred to as aPS/PT hereinafter), which reacts only with prothrombin forming a complex with phosphatidylserine in blood of a patient (also referred to as PS/PT hereinafter), closely correlates with LA. However, relation of antibodies directed to blood coagulation factor/phospholipid complexes other than aPS/PT to LA cannot be ruled out. Thus, it remains unknown whether a monoclonal antibody that does not bind to these complexes, aPT/PS, inhibits blood coagulation or can be used as a standard for LA.